One or more aspects relate, in general, to processing within a computing environment, and in particular, to memory accesses within the computing environment.
System configurations include physical memory used to store applications and data. The amount of physical memory is fixed and often inadequate to support the needs of users. Therefore, to provide additional memory or at least the appearance of additional memory, a memory management technique, referred to as virtual memory, is utilized. Virtual memory uses virtual addressing, which provides ranges of addresses that can appear to be much larger than the physical size of main memory.
To access main memory in a system configuration that includes virtual memory, a memory access is requested that includes an effective address. The effective address is translated into a real address used to access the physical memory.
Translation is performed using an address translation technique. Several address translation techniques are available. For instance, in PowerPC systems offered by International Business Machines Corporation, an effective address is translated to a corresponding real address by way of page table entries found by selecting an effective segment identifier (ESID) table entry associated with the effective address, and using the entry to locate a group of page table entries by way of a hashing algorithm. In a further example, in the z/Architecture, also offered by International Business Machines Corporation, an effective address is translated to a corresponding real address by way of a hierarchy of translation tables. Translation tables are indexed by a portion of the effective address to find the address of the next translation table of the hierarchy until a real (or absolute) address is obtained. Both address translation techniques provide advantages to their respective operating systems.